Thin Film Transistor-Liquid Crystal Display (TFT-LCD) is one of the promising industries nowadays. The technology for manufacturing displays has been advanced with an unimaginable speed. However, during the race between technology advancement and consumers' pursuit of higher visual satisfaction, a major problem, MURA, pixel defects or wide-area pixel defects (also known as Mura defects), surfaces itself and bothers the manufacturers a great deal. There are a lot of reasons for causing the Mura defects. The reasons are generally classified into two categories; the cell unit defects and the backlight unit defects. Still, the residual stress remained in the glass substrate is also one of the reasons that the manufacturers cannot overlook. As a result of this, measurement of the residual stress has also become important.
Photoelasticity is a real-time, full-field, high sensitive and non-destructive stress measurement method. Mainly, it uses the birefringent material at issue or the temporary birefringent material at issue to examine the residual stress remained in the material at issue. The former employs the characteristics of the difference between the refraction index of the direction of the optical axis of the material at issue and the refraction index of the direction of the orthogonal optical axis. Yet, the latter uses the birefringent feature of the material at issue under a stress.
U.S. Pat. No. 5,400,131 discloses a method for measuring stress in an object of birefringent materials. The stress has both magnitude and direction. The method comprises steps of passing polarized light of first, second and third wavelengths through an object and an analyzer to produce respective fringe patterns, measuring and recording intensities of light for each wavelength emitted from the analyzer for multiple positions in the respective fringe patterns and combining the recorded intensities for the first, second and third wavelengths to form a ramp map having discontinuities at predetermined values of stress and converting the ramp map to a stress map indicating the magnitude of the stress in the object as a function of position within the object, which cannot quantify residual stress precisely because each predetermined value comes from a different source.
In the conventional photoelastic method, the stress-optic law, by analyzing light intensity and extracting the fringe order of isochromatic fringe pattern, is followed when used in quantifying the material stress. Then from the predetermined or known stress-optic coefficient or known wavelength of a light source, the material stress value is then calculated. As introduced, the quantifying method for measuring material stress has to use different measurement systems to acquire stress-optic coefficient and wavelength, which will lead to accumulation of errors from different systems and is not appropriate for quantifying material stress which requires precise measurement. In addition, those required stress-optic coefficient and wavelength may not be independent from each other such that when the material stress value is low, i.e., unknown stress and/or residual stress, the final calculation of the stress will not be correct and is not appropriate for measuring material stress. In addition, the conventional photoelasticity often uses full-field image acquisition equipment and that is not available to present different light intensities from different wavelengths of light.
Furthermore, in the conventional stress measurement method, a comparison between spectrum of the known stress in the database and spectrum of an unknown stress is conducted. Once there is a match, the unknown stress then becomes known. However, this comparing method requires lots of comparing time. And if the spectrum of the unknown stress does not match to any of spectrums of the known stresses in the database, an error occurs. As a complete and continuous database corresponding relationship between stress and spectrum is not available, this measurement method can only deal with the stresses whose values are just equal to the values of the known stresses in database for matching. Once there is a mismatch, the result will lead to erroneous conclusion for there is not known stress for calculating the unknown stress. Increasing the data number of the known stress in the database may improve this dilemma, but it is real not practice.
Further, all the above techniques measure the fringe order or the retardation. All of which are conducted by way of stress-optic law to convert into stress value so that the stress-optic coefficient of the material to be tested should be known. This requirement is not practical in the on-line real-time inspection. In addition to that, the measurement error of the stress-optic coefficient could be accumulated in the calculation of the stress, which ultimately leads to false stress result.
Therefore, it is crucial to design a method and apparatus to directly and quantitatively measure stress and residual stress in a material without first measuring stress-optic coefficient by using fine information of different colors of light and systematically establishing a database between stress and the corresponding spectrum.